Processes in the brain and other organs involving serotonin as a neurotransmitter have been a major field of pharmacological research for some decades. A large number of processes which depend on serotonin have been identified, and numerous therapeutic compounds which affect such processes are in widespread use. More than a dozen receptors which are acted upon by serotonin have been identified. Some of the receptors' physiological mechanisms have been identified, and others are still the subject of extended and active research.
One of the more recently identified serotonin receptors is known as 5-HT.sub.4. Bockaert J., Fozard J., Dumuis A., et al., "The 5-HT.sub.4 Receptor: A Place in the Sun", Trends Pharmacol. Sci., 13, 141, 1992. Therapeutic methods making use of the 5-HT.sub.4 receptor have been held back by the lack of compounds which affect the 5-HT.sub.4 receptor without substantial effect at other receptors. The present invention provides a series of new pharmaceutical agents which have a high affinity at the 5-HT.sub.4 receptor.